Our invention relates to semiconductor devices, and more particularly to a high-voltage semiconductor device having a rectifying barrier or barriers as in the form of a Schottky barrier, a p-n junction, or both. Still more particularly, our invention deals with an improved high-voltage blocking technique in such semiconductor diodes or rectifiers. Our invention also specifically concerns a method of making such semiconductor devices.
Among the high-voltage blocking techniques heretofore suggested for Schottky-barrier or p-n junction semiconductor devices are the field plate, the field-limiting ring, and a combination of both. All these contrivances share the same objective of making the voltage-withstanding capability at the periphery of the main rectifying barrier as close as possible to that at its midportion. The listed prior art techniques are unsatisfactory in either the simplicity of construction, the ease of fabrication, or the extent to which the desired objective is accomplished.
Our own solution to this problem is what we call the RESP (REsistive Schottky barrier field Plate). We have described and claimed an adaptation of the RESP for Schottky-barrier semiconductor devices in our copending U.S. patent application Ser. No. 277,333 filed Nov. 29, 1988, and its adaptation for p-n junction semiconductor devices in our copending U.S. patent application Ser. No. 319,951 filed Mar. 7, 1989.
In these prior applications the RESP takes the form of a thin layer of titanium oxide in direct contact with a semiconductor substrate. The titanium oxide layer can be formed by thermally oxidizing a preformed titanium layer in situ on the semiconductor substrate. Its sheet resistance is constant throughout its expanse. A reverse current flows through the RESP upon application of a reverse voltage to the semiconductor device, with the consequent creation of a potential gradient therein that serves to mitigate field concentrations at the periphery of the main rectifying barrier. The overall voltage-withstanding capability of the device is thus materially improved.
The semiconductor specialists might be tempted to identify the RESP with the familiar high-resistivity field plate. The RESP distinctly differs from the conventional high-resistivity field plate, which overlies an insulating region, in making direct contact with a semiconductor region to form a Schottky barrier. This difference makes the RESP far superior to the high-resistivity field plate in voltage-withstanding capability and the stability of operation at elevated temperatures, as we have confirmed by experiment.
We must, however, admit a weakness of the RESP construction. As we stated in the U.S. patent application Ser. No. 319,951 cross-referenced above, a series of preliminary breakdowns take place at limited parts of the RESP periphery upon application of a reverse voltage, before the final breakdown of the device. Being limited by the resistivity of the RESP, the reverse current flowing as a result of the preliminary breakdowns is not so serious as to adversely affect the overall voltage-blocking capability of the device. The preliminary breakdowns are nevertheless undesirable as it involves the flow of reverse current, no matter how negligible its magnitude may be.